1. Field of the Invention
The present invention relates to a method of passivating germanium. More specifically, the present invention relates to a method of carburizing germanium to form germanium carbide.
2. State of the Art
One significant reason for the wide use of silicon, rather than germanium, as a semiconductor substrate is the ease with which silicon is passivated. The silicon oxidizes, producing silicon dioxide, which is a good electrical insulator and passivator. The silicon dioxide also acts as a barrier, preventing impurities from penetrating the silicon without affecting the properties of the silicon. Alloys of silicon and germanium are used in high-speed bipolar transistor structures in high frequency wireless radio frequency (“RF”) circuits. In addition, alloys of silicon, carbide, and germanium are used in high-power, high temperature semiconductor devices. However, in many applications, such as in high speed devices, germanium is a more suitable semiconductor substrate than silicon or silicon alloys because the electron mobility of germanium is higher. Germanium also has a direct transition that is only slightly higher in energy than the indirect band gap. As a consequence, germanium has a higher absorption coefficient than silicon, making germanium desirable in many optoelectronic or photovoltaic applications.
While germanium also forms a native oxide, germanium oxide is a poor insulator, is soluble in water and other solvents typically used to process semiconductor devices, and is volatile at elevated temperatures typically used to process the semiconductor devices. Therefore, germanium oxide is not a good passivator for a germanium surface. As disclosed in U.S. Pat. No. 4,589,006 to Hansen et al., a diode formed from a germanium substrate is passivated with a layer of hydrogenated amorphous germanium or hydrogenated amorphous silicon. The layer is formed by sputtering the germanium or silicon in a low pressure atmosphere of hydrogen and a noble gas.
Hydrogenated amorphous silicon is a widely studied material. It has been determined that the band gap energy of the hydrogenated amorphous silicon depends on the degree of short range order in the material. The band gap energies of amorphous materials are not well defined or well known but are generally higher than those of corresponding crystalline materials. For instance, the band gap of crystalline silicon is 1.2 eV but that of amorphous silicon is up to 2.0 eV.
U.S. Pat. No. 6,794,255 to Forbes et al., which is commonly assigned to the assignee of the present invention, discloses forming silicon carbide by carburizing silicon. The resulting silicon carbide is preferably amorphous. The silicon is carburized using microwave plasma-enhanced chemical vapor deposition (“MPECVD”). The silicon carbide is used as an insulating dielectric layer in a field effect transistor (“FET”).
Band diagrams for crystalline silicon and crystalline silicon carbide are shown in FIGS. 1 and 2, respectively. The band gap of crystalline silicon is 1.2 eV, the electron affinity is 4.2 eV, and the electron barrier energy is 3.3 eV while the band gap of crystalline silicon carbide is 2.1 eV, the electron affinity is 3.7 eV, and the electron barrier energy is 2.8 eV. In contrast, amorphous hydrogenated silicon carbide deposited by very high frequency plasma enhanced chemical vapor deposition has a band gap of up to 3.4 eV. As such, the amorphous hydrogenated silicon carbide has a greater band gap than that of the crystalline silicon carbide (2.1 eV). The amorphous silicon carbide deposited on silicon has been shown to have a low surface recombination velocity and provides good passivation on silicon.
Germanium carbide films have been deposited or grown by chemical vapor deposition (“CVD”), plasma assisted CVD, molecular beam epitaxy (“MBE”), glow-discharge decomposition, RF reactive sputtering, or electron cyclotron resonance (“ECR”) plasma processing. Electrical and optical properties of the resulting films depend on the process of preparing the germanium carbide films and the processing conditions that are used. Booth et al., “The Optical and Structural Properties of CVD Germanium Carbide,” J. de Physique, 42(C-4) 1033-1036 (1981) discloses forming germanium carbide films by CVD. The germanium carbide films have polycrystalline germanium clusters distributed in a GeyCz material and, therefore, are neither crystalline nor amorphous. As disclosed in Chen et al., “Electrical Properties of Si1-x-yGexCy and Ge1-yCy Alloys,” Journal of Electronic Materials, 26(12):1371-1375 (1997), Ge1-yCy alloys that are rich in germanium are deposited on n-type silicon substrates by MBE. The Ge1-yCy alloys have improved crystalline quality and reduced surface roughness compared to pure germanium. Tyczkowski et al., “Electronic Band Structure of Insulating Hydrogenated Carbon-Germanium Films,” Journal of Applied Physics, 86(8):4412-418 (1999) discloses forming hydrogenated carbon-germanium films by plasma assisted CVD from tetramethylgermanium in a RF glow discharge. After annealing, the hydrogenated carbon-germanium films have a band gap energy as high as 7.1 eV and an electron affinity of 1.2 eV.
Amorphous, hydrogenated germanium carbide has been deposited by a variety of techniques. U.S. Pat. No. 4,735,699 to Wort et al., Liu et al., “Structure and Properties of Germanium Carbide Films Prepared by RF Reactive Sputtering in Ar/CH4,” Jpn. J. Appl. Phys., 36:3625-3628 (1997), Yu et al., “Asymmetric Electron Spin Resonance Signals in Hydrogenated Amorphous Germanium Carbide Films,” Phys. Stat. Sol. B, 172(1):K1-K5 (1992), Shinar et al., “An IR, Optical, and Electron-Spin-Resonance Study of As-deposited and Annealed a-Ge1-xCx:H Prepared by RF Sputtering in Ar/H2/CH3H8” J. Appl. Phys., 62(3):808-812 (1987), M. Kumru, “A Comparison of the Optical, IR, Electron Spin Resonance and Conductivity Properties of a-Ge1-xCx:H with a-Ge:H and a-Ge Thin Films Prepared by R. F. Sputtering,” Thin Solid Films, 198:75-84 (1991), and Kelly et al., “Application of Germanium Carbide in Durable Multilayer IR Coatings,” SPE Vol 1275 Hard Materials in Optics, (1990) disclose forming amorphous, hydrogenated germanium carbide films on silicon and glass substrates by RF reactive sputtering. The amorphous, hydrogenated germanium carbide films are formed from a germanium target using mixtures of argon and methane or an inert gas and a halocarbon gas. The resulting films are smooth, featureless, and have an amorphous structure. Increasing the carbon content in the amorphous, hydrogenated germanium carbide films increased the hardness of the films. The atomic ratio (Ge/C) of the amorphous, hydrogenated germanium carbide films decreased by increasing the gas flow ratio.
Microcrystalline germanium carbide alloys are disclosed in Herrold et al., “Growth and Properties of Microcrystalline Germanium-Carbide Alloys,” Mat. Res. Soc. Symp. Proc., 557:561-566 (1999). The microcrystalline germanium carbide alloys are formed at low temperatures (300° C.-400° C.) on glass, stainless steel, or crystalline silicon substrates by a reactive hydrogen plasma beam deposition technique. The microcrystalline germanium carbide alloys are grown using an ECR reactor. Up to 3% carbon is incorporated into the microcrystalline germanium carbide alloys. The microcrystalline germanium carbide alloys have a high degree of crystallinity and a grain size on the order of a few tens of nm. The best crystallinity is obtained on conducting substrates, indicating the importance of hydrogen ion bombardment in promoting crystallinity. The defect density was low at the tested carbon content. In Herrold et al., “Growth and Properties of Microcrystalline Germanium-Carbide Alloys Grown using Electron Cyclotron Resonance Plasma Processing,” Journal of Non-Crystalline Solids, 270:255-259 (2000), higher temperatures (350° C.-400° C.) are used to grow the microcrystalline germanium-carbide alloys with hydrogen dilution and ion bombardment. The microcrystalline germanium-carbide alloys have up to 2% carbon incorporation. Optical absorption curves of the microcrystalline germanium-carbide alloys parallel that of crystalline germanium. In addition, the microcrystalline germanium-carbide alloys have increased band gaps with increasing carbon incorporation. At comparable band gaps, the absorption coefficient of the microcrystalline germanium-carbide alloys is larger than that of crystalline silicon. Microcrystalline hydrogenated germanium carbide films formed by RF reactive sputtering or ECR plasma processing have a low carbon concentration, such as 4%. Therefore, the microcrystalline hydrogenated germanium carbide films have a band gap energy that is close to that of silicon (1.2 eV).